1. Field of the Invention
The present invention relates to a multistage vacuum pump unit including a plurality of separate single-stage pumps connected in series with each other by exhaust pipes each connecting a suction port of a single-stage pump with an exhaust port of another of the adjacent single-stage pumps subsequent to the single-stage pump, motors for driving the separate single-stage pumps respectively, driving means for varying a revolution of the pump that is at least in contact with an atmospheric side, and driving current detection means for detecting a driving current of the motor for driving the pump that is in contact with the atmospheric side. The multistage vacuum pump unit thus constructed improves a backing pressure-side vacuum level at an exhaust port of the vacuum-side single-stage pump that is connected in series with the subsequent single-stage pump, prevents a pumping speed from decreasing, and inhibits a driving force (power) from increasing.
2. Description of the Prior Art
FIG. 9 illustrates a first conventional multistage vacuum pump unit disclosed in Japanese Patent Application Laid-Open No. Hei 5-240181. This pump unit is provided with two separate pumps P1, P2 driven by separate motors M1, M2 respectively. The motors M1, M2 are activated via an ac power that is supplied from inverters I1, I2. The inverters I1, I2 are controlled by a controller CR.
FIG. 10 illustrates a second conventional multistage vacuum pump unit disclosed in Japanese Patent Application Laid-Open No. Hei 7-305689. This pump unit is provided with a plurality of Roots pumps R1 to R4 accommodated in separate casings C respectively. Pump chambers PC are connected in series with each other by exhaust pipes E2 to E4. Drive shafts of rotors RT allocated to the respective Roots pumps R1 to R4 are arranged separately from each other, so that the Roots pumps R1 to R4 are driven at different revolutions using a belt or a pulley (not shown).
In the case where the first conventional multistage vacuum pump unit is practically employed, the pumps P1, P2 are not supplied with loads uniformly depending on a flow rate of gas. It is thus necessary to set the optimum revolutions of the pumps P1, P2 in accordance with the flow rate of gas. However, the first conventional multistage vacuum pump unit is not provided with any detection means for detecting temperatures, pressures and current signals or any control circuit required to constitute a feedback system for setting the optimum revolutions. That is, the first conventional multistage vacuum pump unit is unable to set the optimum revolutions of the pumps in accordance with the flow rate of gas, so that the pumping speed is highly susceptible to a vacuum level.
The second conventional multistage vacuum pump unit requires setting a vacuum-side revolution to a higher value, using pumps of an equal capacity, and setting a rotor clearance less than 0.1 mm. Accordingly, the exhaust pipes E1 to E4 for the respective pumps are provided with expensive vacuum gauges, which causes a problem of cost enhancement.